This is very much a LAN problem...
My router can ping via IPv6:
But my current IPv6 table does not show a global address:
Even if the WAN connection has one:
Meaning I get gived a ULA address...
Now solved (cf: https://community.bt.com/t5/Home-setup-Wi-Fi-network/IPv6-on-a-Draytek-router/td-p/2331083)
Thanks @WSH & @licquorice 🙏